1. Mouth and Salivary Digestion
Mechanical digestion begins with mastication. Three salivary gland pairs (parotid, sublingual, submandibular) secrete saliva containing salivary amylase (ptyalin), which hydrolyses starch → maltose + isomaltose + limit dextrins at optimal pH 6.8. The bolus is propelled by peristalsis. Key fact: salivary amylase is inactivated by stomach acid and contributes minimally to total starch digestion.
2. Stomach
J-shaped muscular organ with rugae (expansion folds). Cell types: parietal/oxyntic cells → HCl (pH 1.5–2.0); chief/zymogenic cells → pepsinogen (→ pepsin, activated by HCl); goblet cells → mucus (protection). HCl functions: (1) bacterial killing, (2) pepsinogen activation, (3) optimal pH for pepsin. Chief cells also produce gastric lipase (short-chain fats, pH 4–5). Output: chyme through pyloric sphincter. Gastrin hormone amplifies gastric secretion.
3. Duodenum — The Digestive Hub
Receives pancreatic juice + bile simultaneously.
- Pancreatic juice: proenzymes (trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase) + active enzymes (lipase, amylase, nucleases). Activation: enterokinase (duodenal mucosa) → trypsinogen → trypsin → cascade.
- Bile: produced by liver, stored in gallbladder, released by CCK. Contains bile SALTS (no enzymes) → fat emulsification only.
- Hormones: Secretin (acid → bicarbonate), CCK (fats + AA → bile + enzymes), GIP (fats + glucose → inhibit acid + insulin).
4. Jejunum and Ileum — Absorption Hub
Structural adaptations: villi + microvilli (brush border) → 600-fold surface area amplification. Brush border enzymes: maltase (maltose → 2 glucose), sucrase (sucrose → glucose + fructose), lactase (lactose → glucose + galactose), aminopeptidase + dipeptidases (peptides → amino acids), nucleotidases + nucleosidases (nucleotides → bases + sugars). Absorption: glucose + amino acids → active transport → blood capillaries → portal vein. Fats → passive → chylomicrons → lacteals → thoracic duct → left subclavian vein.
5. Large Intestine
Characterised by haustra (sacculations). Functions: (1) water absorption (~1.3–1.8 L/day, osmosis), (2) mineral absorption, (3) bacterial synthesis of vitamins B and K, (4) faeces compaction and storage. Critical point: NO nutrient (glucose, amino acid, fat) absorption here. Defecation controlled by medullary reflex.
6. Liver and Pancreas as Accessory Organs
Liver: synthesizes bile, processes portal blood (absorbed nutrients → metabolism), produces albumin, detoxifies, excretes bilirubin. Pancreas (exocrine): secretes pancreatic juice (enzymes + bicarbonate) into duodenum via pancreatic duct.
7. Hormonal Regulation Summary
| Hormone | Source | Trigger | Primary Action |
|---|---|---|---|
| Gastrin | G-cells, pyloric stomach | Food/proteins | HCl + pepsinogen ↑ |
| Secretin | S-cells, duodenum | Acidic chyme | Pancreatic bicarbonate ↑ |
| CCK | I-cells, duodenum | Fats + amino acids | Bile + pancreatic enzymes ↑ |
| GIP | K-cells, duodenum | Fats + glucose | Gastric acid ↓, Insulin ↑ |
8. Nutritional Deficiency Diseases
- Kwashiorkor: protein deficiency (adequate calories) → low albumin → oedema, distended belly, skin lesions
- Marasmus: total calorie + protein deficiency → extreme wasting, emaciation, NO oedema